1. Field of the Invention
The present invention relates to an optical pickup that projects a laser beam onto an optical disc for reproducing or recording information. In particular, the present invention relates to an optical pickup having lenses with different numerical apertures for recording or reproducing information on a CD, a DVD and a BD, as well as a manufacturing method of the optical pickup.
2. Description of Related Art
An optical disc apparatus records or reproduce information on the optical disc as a recording medium by projecting a laser beam onto the optical disc from the optical pickup. If the optical disc apparatus supports a CD (Compact Disc) medium and a DVD (Digital Versatile Disc) medium, the optical pickup is equipped with a laser light source that emits infrared laser (having a wavelength of approximately 780 nm) for a CD and red laser (having a wavelength of approximately 650 nm) for a DVD. In addition, the optical pickup is equipped with an objective lens that condenses the laser beam to make a circular laser spot on a track formed on a recording surface of the optical disc.
A diameter of the laser spot is proportional to a wavelength of the laser beam and is inversely proportional to a numerical aperture (NA) of the objective lens. Since a width of the track of the CD medium is different from that of the DVD medium, the diameter of the laser spot for recording or reproducing information is also different between the two media. Therefore, two objective lenses having different numerical apertures are necessary. However, a single objective lens can support both the CD medium and the DVD medium by modifying a shape of the objective lens.
In addition, the optical pickup includes an objective lens holder that holds the objective lens. The objective lens holder is moved in a radial direction (tracking direction) of the CD medium or the DVD medium, and in the direction approaching to or separating from the media (focusing direction). This movement enables the laser beam to be projected on the track of the optical disc accurately.
Recently in addition, a BD (Blu-Ray Disc) has become available, which has the same diameter as the DVD and larger storage capacity than the DVD. Since a BD medium has higher recording density than the DVD medium, a track width of the BD medium is much smaller than that of the DVD medium. Further, the laser spot for the BD should be smaller, too. Therefore, a laser light source that emits blue laser having a shorter wavelength (approximately 405 nm) than the red laser is used for recording or reproducing information on the BD medium.
On the other hand, the objective lens for a CD/DVD has a small NA that cannot condense the blue laser sufficiently, so it is difficult to use it for recording or reproducing information on the BD medium correctly. Furthermore, if an objective lens for a BD is used, it is difficult to condense the infrared laser or the red laser appropriately. Therefore, it is proposed to provide two objective lenses having different numerical apertures, i.e., an objective lens for a CD/DVD and an objective lens for a BD to the lens holder that holds the objective lens.
Using such an optical pickup provided with two objective lenses having different numerical apertures, the laser beam can be projected onto the optical discs having different track widths accurately so that recording and reproducing of information can be performed accurately.
In the case of the above-mentioned optical pickup having two different objective lenses, even if the optical pickup is adjusted so that the optical axis of the laser beam becomes orthogonal to a recording surface of the optical disc with respect to a tilt angle of one of the objective lenses, the optical axis of the laser beam going out from the other objective lens is not always orthogonal to the recording surface of the optical disc. It is because that a tilt error of the objective lens when it is attached to the lens holder, a shape error of the attaching portion, or the like is not always the same between the both objective lenses.
Therefore, the objective lens is rotated around its central axis in the objective lens holder so that adjustment of a tilt of the objective lens (tilt angle adjustment) can be performed in many cases. In this case, before the objective lens is fixed to the objective lens holder, the objective lens is rotated for the adjustment of a tilt angle (see JP-A-2000-322756 for example).
However, the objective lens is usually formed to have no bump or dip, so it is hardly rotated even if a jig like tweezers is used. Even if it can be rotated, very high technique is necessary to perform a delicate adjustment. Therefore, JP-A-2006-064886 discloses a structure in which a recess is formed at the periphery of the objective lens so that a jig is inserted in the recess so as to rotate the objective lens.
In addition, JP-A-H6-168456 discloses a structure in which a spacer with protrusions is used for attaching the objective lens. The protrusions are disposed every 90 degrees alternately on the front and the rear sides and are used for adjustment of a tilt angle of the objective lens.
However, if the recess is formed at the periphery of the objective lens as the invention shown in JP-A-2006-064886, a manufacturing process of the objective lens becomes complicated. If the recess is formed by a post process of the manufactured objective lens, stress or heat due to the post process may cause distortion in the objective lens, so it is difficult to maintain high accuracy of the objective lens. Since a complicated process or a manufacturing method is necessary, manufacturing cost may be increased.
In addition, in the case of the invention described in JP-A-H6-168456, flexibility of adjustment of a tilt angle of the objective lens is low. Therefore, it is difficult to adjust so that two objective lenses have substantially the same tilt degree. In addition, even if they can be adjusted, it is necessary to use many spacers. Therefore, cost increases and weight of the objective lens holder increases so that recording or reproducing accuracy of the optical pickup may be lowered. In addition, since weight of the objective lens holder increases, a larger force is necessary for supporting and driving it, resulting in increase of manufacturing cost and running cost.